Heretofore, in a magnetostrictive sensor structure 1, as shown in FIG. 9, a thin plate 2 made of a ferromagnetic material (magnetostrictive material) such as permalloy is provided with four small holes 3, arranged in cross directions coil wires are passed through the holes 3 in vertical and horizontal directions to form an excitation coil 4 and an output coil 5, located perpendicular to each other and 45 degrees from a force application direction.
When no force is applied to the thin plate 2, as shown in FIG. 10, magnetic flux distribution direction of the excitation coil 4 is parallel to the winding direction of the output coil 5; no coupling is present between the excitation coil 4 and the output coil 5; and a gain voltage (output voltage from the output coil 5) does not change.
On the other hand, as shown in FIG. 11, when a force is applied to the thin plate 2, the permeability decreases in the force applying direction (because a tensile stress is generated) and increases in the perpendicular direction (because a compressive stress is generated), and magnetic flux distribution of the excitation coil 4 changes in a 45-degree direction.
Consequently, part of the changed magnetic flux distribution crosses the output coil 5 to induce a voltage proportional to the applied force from the output coil 5, whereby the voltage can be utilized to obtain, for example, an output signal for load detection.
The magnetostrictive sensor structure 1, as shown in FIG. 12, for example, is welded with cylindrical bodies 6 at both sides of the thin plate 2 so as to form a magnetostrictive sensor 7. The magnetostrictive sensor 7 is engaged at a desired measured location through the cylindrical bodies 6.
Recently, to prevent large-sized vehicles such as trucks from being overloaded, it was considered that a load measuring apparatus equipped with the magnetostrictive sensor 7 be incorporated directly in the vehicle itself so that the driver or the transportation business could easily determine the load weight.
For example, Japanese Patent Applications 07-273492, 07-273524, and the like applied by the present applicant proposed a construction as shown in FIG. 13.
This construction incorporates a load measuring apparatus comprising the magnetostrictive sensor 7 on a truck using a so-called tandem axle structure which has a suspension structure using a leaf spring and two axles provided at the rear axle side to reduce pressure to the road surface.
At the front axle 11 side, a leaf spring 13 of a suspension 12 of the vehicle and a bracket 15 at the bed frame 14 side are linked by a shackle pin 17 through a bushing 16; a shaft hole 18 is provided along the axial direction on the center line of the shackle pin; and the magnetostrictive sensor 7 having the magnetostrictive sensor structure 1 is engaged in this shaft hole 18.
At the rear axle 21 side, a trunnion bracket 22 mounted to the bed frame 14 is engaged with a trunnion shaft 23, and the trunnion shaft 23 is supported on a spring seat 25 of a leaf spring 24. A shaft hole 26 is provided on the center line of the trunnion shaft 23 along the axial direction, and a magnetostrictive sensor 7 having the magnetostrictive sensor structure 1 is placed in the shaft hole 26.
A strain of the shackle pin 17 due to a load at the front axle 11 side is detected by the magnetostrictive sensor structure 1, and a strain of the trunnion shaft 23 deformed in proportion to the sprung weight of the vehicle at the rear axle 21 side is detected by the magnetostrictive sensor structure 1. Then, these detection signals of the magnetostrictive sensor structure 1 are summed to obtain the vehicle sprung weight, and a calculation is made such as addition of the unsprung weight and the like to measure the load weight.
Since such a vehicle load measuring apparatus is subjected to the vehicle load such as the shackle pin 17 and the trunnion shaft 23 and vibration, and the magnetostrictive sensor 7 is inserted and engaged in a large-diameter shaft having a high strength, a strain received by the magnetostrictive sensor structure 1 of the magnetostrictive sensor 7 is as small as several .mu.m.
Therefore, in order to efficiently detect a small strain in the magnetostrictive sensor 7 used in the vehicle load measuring apparatus, the magnetostrictive sensor structure is practically structured as shown in FIG. 3.
Specifically, four small coil insertion holes 3 are provided in cross directions at the center of the rectangular thin plate 2 comprising a magnetostrictive material, and U-formed cutouts 8 are provided at the right and left sides of the outside of the position of the hole 3 of the upper line and lower line of the rectangular thin plate 2.
However, even with the U-formed cutouts 8, a very small deformation of several to 10 and some .mu.m generated in the large-diameter shaft such as the shackle pin 17 or the trunnion shaft 23 cannot be efficiently detected, thus producing various problems in terms of the sensitivity, accuracy, and stability.
Further, in the magnetostrictive sensor structure 1, coil wires must be inserted and wound in the four holes 3 provided in cross directions to form the excitation coil 4 and the output coil 5. However, the work of winding a coil wire (enameled wire or resin-coated wire) of about 100 .mu.m in wire diameter in the small hole 3 of about 1.phi. is difficult for a robot to perform, and must be achieved manually.
However, when carrying out the winding work manually, a deviation of the tensile strength or winding of a wrong number of turns tends to occur, a uniform magnetic flux distribution cannot be obtained in each product, and a deviation tends to occur in measurement accuracy. Further, because the winding work is carried out manually, the production cost is considerably increased.